As the previous European standards for the use of UHF ISM-frequency band (ISM=Industrial-Scientific-Medical) for RFID (radio frequency identification) applications did not fulfill the requirements of modern systems any more, meanwhile, by order of the European Commission, a proposal for a new standard has been defined, which is published under number EN 302 208 and the title “Electromagnetic compatibility and Radio spectrum Matters (ERM); Radio Frequency Identification Equipment operating in the 865 MHz to 868 MHz band with power levels up to 2 W ”; this document can be downloaded from the web page http://www.etsi.org on the Internet. This new UHF standard EN 302 208 will be harmonized in Europe by mid 2006. In comparison with the previous regulation the essential advantage of this new regulation is a higher permitted transmitting power of 2 W ERP (=Effective Radiated Power) of the reading units in comparison with 0.5 W ERP today. Thus, for the first time in Europe UHF RFID Long Range Systems having a 4 to 6 m range and a large number of transponders is possible, against which previously only systems with ranges of maximum 2.5 m and few transponders could be implemented. Furthermore, the previous restriction of the duration of connection per cycle duration at maximum ten percent (10%) ceases to apply. The new standard also offers 10 channels with 200 kHz bandwidth each, as against a total bandwidth of 250 kHz today.
However, the new regulation also entails problems. For example, the new regulation demands an operating method of the reading units, which is called “Listen Before Talk” (LBT), in order to confront the danger of disturbances of other users in the ISM band, due to the higher permissible transmitting power of the reading units. Listen before talk is understood to mean that each reading unit that would like to communicate with a transponder by transmitting a modulated electromagnetic field should first check whether the channel intended to be used is free, that is, is not already being used by another reading unit in the vicinity. Precisely this means that a reading unit in the listen mode may not receive a signal with a power above −96 dBm. This limit is very stringent and hard to comply with and may lead to considerable restrictions in what is called a dense reader environment. An example of such a dense reader environment is what is called a Dock-Door application, in which at least two delivery gates are directly next to each other, as is usual in logistic applications. Then, trucks are directly unloaded via these typically 3 m broad delivery gates, wherein the goods provided with RFID transponders are scanned by an RFID reading unit during their moving through a respective delivery gate. These delivery gates, which are often arranged only at a distance of 10 cm next to each other, mostly have a respective reading unit and several antennas connected to the reading unit, wherein the antennas are laterally mounted on the delivery gates and directed to each other, so that they radiate to the gate area to be scanned. If, for example, four antennas are arranged per gate (two on either side) then with two delivery gates next to each other, two respective antennas of the adjacent delivery gates are also directed to each other. This can be identified clearly in FIG. 1, where the antennas A1 and A3 of the first delivery gate G1 and the antennas B2 and B4 of the second delivery gate G2 are turned towards each other. Assuming that the reading unit R2 of the delivery gate G2 is active just on the channel 0, as it is transmitting electromagnetic radio signals EB2 or EB4 over the antenna B2 or B4, then if the reading unit R1 were in the listen mode, it would receive these radio signals EB2, EB4 over the antennas A1 or A3, where the power of the received signals EB2 or EB4 would exceed −96 dBm.
Therefore, in compliance with the specifications of the EN 302 208 regulation, the reading unit R1 would have to regard the channel 0 as occupied and could not itself become active on this channel 0. There is a risk that the reading unit R1 additionally regards one or several other channels as occupied, in which the said sidebands lie, for example, the channels 1 and 2 as the reading unit R2, when transmitting, also unavoidably generates electromagnetic signal portions in sidebands outside its own frequency band, because no ideal filtering is possible. Therefore, the reading unit R1 would have to switch over to another channel, for example, the channel 3. As a result of the described problems, the number of usable channels for each further reading unit decreases by at least one channel. But, if delivery gates are now directly next to each other, as is the case in warehouses, in the worst case always only one reading unit can be active, if it intentionally occupies a channel with the main part of the transmitted signals and unintentionally occupies the remaining available channels with the signal portions transmitted in sidebands. Thus, the meaningful operation of such a warehouse with RFID is not possible any more, as only one respective truck could be unloaded while being RFID monitored.